What does atheism have to do with any of those guys?
Everything.
"There can no longer be any good a priori, since there is no infinite and perfect consciousness to think it. It is nowhere written that 'the good' exists, that one must be honest or must not lie, since we are now upon the plane where there are only men. Dostoevsky once wrote 'if God did not exist, everything would be permitted'; and that, for existentialism, is the starting point. Everything is indeed permitted if God does not exist." -Jean-Paul Sarte
Hitler was Lutheran and had the support of the Catholic Church in his mission against Jews
Hitler was raised Lutheran and used the RCC as a way to further his eugenics cause. Being that Germans at that time were predominantly Christian, he used it as a tool of manipulation.
Stalin had a degree from a seminary and certainly enacted plenty of religious doctrine of his own.
Yes, Stalin was preparing for the seminary, lost his faith, severely, and enacted no religious doctrines, but anti-religious doctrines.
Not believing things on the basis of no good evidence, which is what atheism is, has never killed anybody. The millions that you mention fell victim to the same kind of faith-based thinking that typifies religious thinking, even if specific supernaturalism wasn't always prominent in their religion. Certainly these despots were held in religious esteem by the followers.
Which is something I've been saying since I arrived at EvC, but denied up and down by the likes of yourself. Religion doesn't begin or end with the supernatural. Religion exists in the minds of the deeply, so-called, irrelegious.
"... secularist will say that people like Stalin didn't murder others because of a godless worldview, but because they were fanatics. Of course they are trying to head off any implications that secularism and atheism heavy contributed to genocide and atrocities. Here they tackle a straw man when the ball carrier is running around the end.
The point is not that every humanist or atheist will engage in a career of axe murdering as a result their unbelief; but that with God out of the equation, they no longer have a foundation from which to legitimately criticize that which they say is wrong...
The non-believer... cannot be intellectually honest without admitting that the atrocities committed by Stalin's purges, Hitler's concentration camps, Mao's red death, Pol-Pot's killing fields, and others, are consistent with assumptions of naturalism and atheism. For example, when Stalin talks of breaking a few eggs to get an omelet, he is applying a standard of "survival of the fittest" to justify his ruthless purges. We might note that animals out in the forest or jungle appeal to the sharpest teeth and fastest legs in deciding who survives. What did Hitler do but give a militaristic interpretation to the philosophy of Nietzsche?" -Robert Meyer
But Hitler? How does he even make it in your list?
Yes, he should not have made that list in any classic sense, since he was a mystic of sorts. But his beliefs were garnered by Nietzschean philosophy while the Nazi's danced to the music of Wagner (pronounced - Vogner).
"It is better to shun the bait, than struggle in the snare." -Ravi Zacharias